Generally, a photolithography process (widely applied during the manufacture of semiconductor devices) utilizes a photo mask which has a portion for transmitting light and a portion for blocking the light to provide the semiconductor device shaped as desired. Such a photo mask is formed by a shielding pattern and a transmitting pattern to allow for selective exposure.
However, a light diffraction phenomenon becomes problematical with the increase of pattern density, which restricts improvement of resolution.
Therefore, a process for increasing the resolution by using a phase shifting mask has been studied in various fields.
A technique for utilizing the phase shifting mask combines a transmitting region which transmits light unaltered with a shifting transmitting region which shifts the light by 180.degree. as it transmits the light thereby preventing degradation of resolution between the shielding pattern and transmitting region.
In association with such a phase shifting mask, modified masks applying a phase difference of light have been suggested to expand the optical resolution limit.
Starting from a Levenson's alternate type phase-shifting mask, a rim type phase shifting mask has been suggested by Nitayama et al. for improving the resolution limit of a contact hole.
Hereinbelow, the conventional phase shifting mask will be described with reference to accompanying drawings.
FIGS. 1A to 1D are sectional views showing a process for manufacturing one example of a conventional phase shifting mask which is of the alternate type.
Briefly, the alternate type phase shifting mask transmits through two patterns except where the interposed shielding pattern exists. The light having passed through the two transparent patterns has opposite amplitudes.
First, as shown in FIG. 1A, an etch-stopper layer 2 and a shielding layer 3 are sequentially formed on a transmitting substrate 1. At this time, shielding layer 3 is generally formed of chrome and has at least a thickness sufficient to obtain a perfect shielding effect.
Referring to FIG. 1B, a resist 4 is deposited onto shielding layer 3, and is patterned via an electron beam radiation and development process. Then, shielding layer 3 is selectively etched via an etching process using patterned resist 4 as a mask, thereby providing a shielding layer pattern 3a having a plurality of opening areas 5.
As shown in FIG. 1C, a phase shifting layer 6 is formed on the whole surface of the resultant structure including shielding layer pattern 3a. Step coverage problems arise where the phase shifting layer 6 fills the apertures in the shielding layer pattern 3a.
Referring to FIG. 1D, phase shifting layer 6 is selectively removed to be alternately formed on opening areas 5.
FIGS. 2A to 2C are sectional views showing a process for manufacturing another example of the conventional phase shifting mask which also is of the alternate type.
Referring to FIG. 2A, an etch-stopper layer 2, a phase shifting layer 6 and a shielding layer 3 are sequentially formed on a transmitting substrate 1.
As shown in FIG. 2B, a resist 4 is deposited onto shield layer 3, and is patterned via the electron beam radiation and development process. Then, shielding layer 3 is etched via the etching process using patterned resist 4 as a mask, thereby forming a shielding layer pattern 3a having a plurality of opening areas 5.
Referring to FIG. 2C, phase shifting areas are alternately defined on plurality of opening areas 5. Thereafter, phase shifting layer 6 of opening areas 5, excluding opening areas 5 defined by the phase shifting region, is selectively removed. Then, the phase shifting mask having opposite phases of light having passed through adjacent opening areas 5 is formed.
FIG. 3 shows a profile diagram representing intensity of the light having passed through the opening areas of the conventional phase shifting mask shown in FIG. 2C. Here, the intensity profile of the light having passed through the opening area formed with the phase shifting layer and the opening area without being formed with the phase shifting layer is steep.
In other words, the amplitude of the light having passed through the adjacent opening areas is exposed under the opposite state to prevent appearance of the abnormal pattern due to a side lobe at the shielding layer pattern region where two opening areas overlap with each other.
FIGS. 4A to 4D are sectional views showing a process for manufacturing still another example of the conventional phase shifting mask, which is the rim type phase shifting layer suggested by Nitayama et al. for improving the resolution limit of a photoresist hole.
In a rim type phase shifting mask, the phase shifting region is formed to have a phase opposite to the phase of light that has passed through the transmitting region onto a rim portion of the transparent area, which is the opening area between shielding regions. Here, the light having passed through the phase shifting region induces an offset interference with the light having passed through the transmitting region so that the abnormal pattern caused by a side lobe is prevented and the precise photoresist hole can be realized.
First, as shown in FIG. 4A, a shielding layer 11 and a resist 12 are sequentially deposited on a transmitting substrate 10. Then, a region forming the shielding layer pattern is defined via the electron beam radiation and development process to pattern the resist 12. Shielding layer 11 is selectively removed via the etching process using patterned resist 12 as a mask to form a shielding layer pattern 11 having a plurality of opening areas 13. The shielding layer pattern 11 is formed of chrome.
Referring to FIG. 4B, after resist 12 is removed, a poly-methyl-methacrylate (PMMA) layer to be used as a phase shifting layer 14 is deposited on the whole surface of the resultant structure including shielding layer pattern 11, of which the rear side is exposed with ultraviolet rays.
In FIG. 4C, the PMMA to be used as phase shifting layer 14 is developed. At this time, since the PMMA is of a positive type, only the portion radiated by the ultraviolet rays is developed to leave the PMMA on shielding layer pattern 11. That is, only the PMMA formed onto opening area 13 is developed.
Referring to FIG. 4D, shielding layer pattern 11 at the rim portion of phase shifting layer 14 is partially removed by using a wet etching method. At this time, a width W of phase shifting layer 14 corresponding to the range of eliminating shielding layer pattern 11 does not correspond in proportion to the desired pattern resulting from the light transmission, rather its purpose is to sharpen the profile of light having passed through opening area 13. It transmits the light phase-shifted by 180.+-.10.degree., which is opposite in phase to the light passing through opening area 13, thereby increasing the resolution of the phase shifting mask.
FIG. 5 shows a profile diagram representing intensity of the light having passed through the rim type phase shifting mask shown in FIG. 4D, in which the light passing through opening area 13 is offset with the light passing through the rim portion of phase shifting layer 14 in contact with opening area 13, so that the light intensity having passed through opening area 13 is steep. In other words, the patterning can be accurately performed in the perpendicular direction when forming the photoresist pattern.
By the conventional alternate type phase shifting mask and rim type phase shifting mask, the side lobe can be prevented and the steep photoresist pattern can be formed as compared with the general photo mask which is employed as the photo-developing reticle. But there are problems as discussed below.
First, since the shielding layer pattern is formed onto the substrate in addition to the phase shifting layer, the bond stress and the like at the contact portion of the shielding layer pattern and phase shifting layer may deform the phase shifting layer pattern.
Second, the shielding layer pattern is formed by using a light-shielding material such as chrome which exhibits a step coverage problem due to the thickness of the shielding layer pattern. Thus, a phase shifting error can be induced by the step coverage when the phase shifting mask is utilized to expose a wafer.
Third, the wet etching process of the shielding layer pattern of the rim type phase shifting mask must undercut the phase shifting layer, which makes it difficult to precisely form the rim portions of the phase shifting layer, thus retarding reliability.
Fourth, the manufacturing process of the phase shifting mask involves several complicated stages with the consequence of it being highly probable that contaminating particles will be generated, thereby degrading the reliability of the phase shifting mask.